A known vertical injection molding machine has a configuration as follows. That is, a tail stock mounted with a mold opening/closing servo-motor is disposed under a fixed die plate having a fixed-side die disposed on the upper surface side thereof. A movable die plate is disposed above the fixed die plate. A movable-side die is disposed on the lower surface side of the movable die plate while an injection unit is mounted and carried on the upper surface side of the movable die plate. The tail stock, the movable die plate, the injection unit, etc. are moved down or up with respect to the fixed die plate by a toggle link mechanism driven to be stretched or folded by a driving force of the mold opening/closing servo-motor. The vertical injection molding machine configured thus can be designed to rotate the fixed-side die having a mold release surface on the top. For example, when two fixed-side dies are provided to be paired, the vertical injection molding machine can be made excellent in mass productivity for insert molding or the like.
However, in the vertical injection molding machine configured thus, massive members such as the tail stock, the movable die plate, the injection unit, etc. are moved up/down integrally by the toggle link mechanism so that such a large weight is applied to the toggle link mechanism. Therefore, there is a problem that it is difficult to detect a foreign matter in the ending period of a mold closing process where the toggle link mechanism is stretched. The detection should be performed for protecting the dies. To this end, in order to diminish the aforementioned large weight, there has been proposed a vertical injection molding machine in which a weight compensation mechanism and a spring are used together to compensate the weight (as disclosed in Japanese Patent Laid-Open No. 2000-263610) or a vertical injection molding machine in which only a spring member is used to compensate the weight (as disclosed in Japanese Patent Laid-Open No. 113964/1998).
However, the technique disclosed in Japanese Patent Laid-Open No. 2000-263610 or Japanese Patent Laid-Open No. 113964/1998 requires an additional mechanism for compensating the weight. Accordingly, there is a problem that the machine cost is increased correspondingly to increase in number of parts. Further, according to the configuration using a spring for compensating the weight, the spring is apt to be damaged due to recurring stress so that the spring must be frequently replaced.